1. Field of the Invention
This invention relates to the use of sulfide ion-providing chemical compounds in gold ore slurries to react with or "kill" residual hypochlorite ions resulting from gold ore slurry chlorination steps. More particularly this invention relates to the use of sodium hydrosulfide (NaHS), sodium sulfide (Na.sub.2 S) and hydrogen sulfide (H.sub.2 S) in processes which recover gold from carbonaceous ores or mixtures of carbonaceous and oxide ores by use of a chlorination step to oxidize continued carbonaceous material and render it incapable of absorbing or complexing with the gold during subsequent cyanidation.
2. The Prior Art
Two general types of gold-containing ores are commonly encountered in gold recovery operations. These types are oxide ores, from which gold or other precious metals are easily extracted by cyanidation techniques, and carbonaceous ores, which contain indigenous organic carbon material and are notoriously refractory to standard cyanidation techniques. Gold-containing carbonaceous ores normally comprise between 0.25 and 3% by weight of carbon. The carbonaceous material in these ores include active carbon and long chain organic compounds. During cyanide leaching processes, carbonaceous materials adsorb gold cyanide complexes [Au(CN).sub.2 --]. The long chain carbon compounds form stable complexes with the gold compounds.
Carbonaceous gold-containing ores are found widely in the United States and in other countries throughout the world. In some instances, carbonaceous ores can be found absent oxide ores, however, the usual case is that mixtures of carbonaceous ores and oxide ores are found together. The fraction of oxide ore in the mixtures of carbonaceous and oxide ores can vary, but such mixtures usually contain up to 70% oxide ore. A characteristic of the ore mixtures is that they are not amenable, because of their carbonaceous ore content, to standard cyanidation techniques. In such cases less than about 50% gold extraction is obtainable from such ore mixtures when they are treated by conventional straight cyanidation methods. For this reason, it is usually more efficient to process the ore mixtures as if they were composed entirely of carbonaceous ore. Ore mixtures containing significant quantities of carbonaceous ore for purposes of this invention are included within the term "carbonaceous ores".
The carbonaceous ores are not amenable to standard cyanidation techniques because the carbonaceous impurities tend to "tie up" the cyanide gold complexes. Research into means of recovering gold from the carbonaceous ores has led to the development of processes utilizing chemical oxidation to render the ores more amenable to cyanidation steps. Oxidation is accomplished by oxygenation with air or other oxygen containing gas, or chlorination with gaseous chlorine, sodium hypochlorite or calcium hypochlorite. The hypochlorites are most effective and yield their best results between about 80.degree. F. to about 120.degree. F.
The above described oxidation processes are usually conducted with the ore slurried in water. A cyanide leach step then follows the oxidation step. When oxidation is accomplished with a chlorine-containing compound, a residual concentration of hypochlorite ions (OCl--) are usually present in the ore slurry at the completion of the chlorination step. Even if chlorine gas were used in the chlorination step, the residual chlorine-containing ion is in the form of hypochlorite ions. This is true because, at the normally prevailing slurry pH values during this point in the process, chlorine gas is rapidly converted to hypochlorite ions as it is dissolved.
Prior art techniques do not include chemical addition to effect removal or reduction of the concentration of hypochlorite ions in the slurry prior to the slurry entering the cyanide leach circuit. Instead, they require long periods in which the hypochlorite ions are allowed to decompose in holding tanks. A requirement in a process for a holding tank greatly adds to the capital expenditure in plant equipment to conduct the process. The techniques of the prior art are, therefore, unnecessarily expensive because they require increased time to process these slurries and additional equipment. The removal of these ions is important because hypochlorite ions react with cyanide ions. This reaction, if allowed to occur, increases the quantity of cyanide compounds required for the reaction and, under certain conditions, can produce undesirable reaction products.
One well-known gold extraction process involving an oxygenation and/or chlorination step prior to a cyanidation leach step is delineated in U.S. Pat. No. 4,289,532 to Matson. The process of this patent utilizes oxygenation and/or chlorination to effect oxidation of carbonaceous material contained in gold-containing ores. When chlorine compounds are used, the process of this patent calls for the ore slurry to be held in a holding tank for 2-3 hours to allow excess hypochlorite to be consumed prior to the ore slurry entering the cyanide leach step. The length of time required to hold the slurry depends on the refractory nature of the ores treated and the amount of chlorine used. The required hold time to eliminate hypochlorite can be decreased by elevating the ore slurry temperature to and maintaining it above about 120.degree. F. Hypochlorite decompose rapidly above this temperature. The cost of heating the slurry to over 120.degree. F. is substantial, especially during cold weather.
Chlorination of carbonaceous gold-containing ore slurries is an effective means of pretreating the ore slurries for subsequent cyanide leaching. The concentration of residual hypochlorite ions described above must be consumed or reduced to very low levels prior to the slurry entering the cyanide leach circuit. Without such removal the hypochlorite ions react with cyanide ions and render the cyanide ions ineffective for gold leaching. It can be seen that a method of rapidly and efficiently consuming excess hypochlorite in chlorinated gold ore slurries would be economically beneficial because the requirement for holding tanks is eliminated and the time to conduct the process is significantly reduced. It is therefore an object of this invention to provide a means of rapidly reducing residual hypochlorite ion concentrations existing in carbonaceous gold ore slurries after oxidation of the carbonaceous materials in the slurries with chlorine compounds. By achieving this objective this invention improves the efficiency of cyanide leach operations on chlorinated gold ore slurries by reducing the concentration of hypochlorite ions entering the cyanide leach step. Other objectives of this invention are to eliminate or reduce the size of holding tanks used in the process and provide an energy savings by eliminating the requirement to elevate the slurry temperature, for decomposition of hypochlorite.